To unload ships carrying bulk cargo in the form of various particulate materials by conventional methods, for instance with the use of grab buckets, involves several disadvantages, int.al. air pollution, spillage of material onto the quay and into the water, irregular unloading rate, sensitivity to poor weather conditions etc. With pneumatic apparatuses difficulties are encountered in unloading particulate goods of specific character and in separating the good conveyed from the air after conveyance.
Systems have attempted to use a vertical screw conveyor but such conveyors in the past had extremely low capacity. The only screw conveyor having a capacity profitable for the unloading of bulk cargoes from ships (where capacities of 500 tons/hour and more are desired) is the type of screw conveyor proposed by the present inventors, i.e. a conveyor equipped with the patented feed device according to especially U.S. Pat. No. 3,596,751 issued Aug. 3, 1971 (but also according to U.S. Pat. No. 3,685,638 issued Aug. 22, 1972).
For a device of this type to be commercially profitable for the unloading of modern bulk cargo ships, it must have a capacity of several hundred tons per hour, preferably 500 tons/hour and more per unit. This can be achieved with large bucket elevators, but these suffer from the disadvantage of being heavy and clumsy. Furthermore, they are unable to reach all points of the cargo hold, and thus large quantities of the cargo are left in the hold. They are therefore unprofitable and cause considerably technical difficulties.
A conventional screw conveyor in vertical position is entirely useless for most materials (zero capacity). Such screw conveyors have been designed only for grain and cement, but their capacities are extremely limited, and they have not found any use in actual practice. On the other hand, a screw conveyor equipped with a feed device according to the abovementioned U.S. Patents can handle quantities per hour that lie from 10 to 25 times higher than those obtainable with conventional screw conveyors for grain. For other goods, the comparison will be still more favorable.
In certain underdeveloped countries (for instance India) conventional vertical screw conveyors according to the above mentioned U.S. Patents are being employed without the devices referred to in the present application, but this means foregoing a number of advantages, and in the highly efficient ports in Europe and the United States, these simple devices cannot compete with other types. The capacity will therefore be lower, and above all there is no possibility of maneuvering the conveyors inside the cargo hold. If one wishes to increase the capacity, the conveyor will be so large and heavy that it cannot possibly be handled without the remaining elements referred to herein and, besides, a tearing means described herein would not be able to function.